There is a growing need to develop miniature spectroscopic sensors capable of real-time sensing of biological and environmental materials. For example, broad-bandwidth, high-spectral-resolution optical detection of human breath has identified multiple important biomarkers correlated with specific diseases and metabolic processes.
As an introduction to planar lightwave Fourier-Transform spectrometers, coupling light into single-mode waveguides is often difficult in most integrated-optic sensors such as arrayed waveguide gratings (AWGs), ring resonators, and Mach-Zehnder interferometers (MZIs). In the case of silicon waveguides, very fine and smooth etching of the taper tips down to ˜80 nm is required to realize a low-loss spot-size converter. The primary advantage of planar lightwave Fourier-transform spectrometers is their relatively easy light coupling and a high optical throughput.
Spatial heterodyne spectroscopy (SHS) is an interferometric Fourier-transform technique based on a modified Michelson interferometer with no moving parts and relies on analysis of stationary interference patterns. In bulk-optic SHS, the mirrors of the Michelson interferometer are replaced by diffraction gratings. This bulk-optic, SHS instrument is more practical than traditional Fabry-Perot and Michelson Fourier-transform spectrometers because of its relaxed fabrication tolerances and ability to correct for interferometer defects and misalignments in data analysis. In addition, the instrument can be field widened without moving parts to achieve high sensitivity.
Early proposals for planar waveguide SHS devices were inspired by known configurations of static Fourier transform spectrometers in bulk optics. A waveguide SHS spectrometer can be formed by interleaving two waveguide phase arrays having opposite dispersion. The interleaved arrays produce two wavefronts that propagate and mutually interfere in the slab waveguide, yielding wavelength-dependent fringes. This is due to the different dispersion of the arrays which makes the wavefronts intersect at different angles for different wavelengths thus forming wavelength-dependent fringe patterns. In general, the input spectrum is related to the fringe pattern via Fourier transform since any input signal can be decomposed into its monochromatic constituents. This interleaved AWG arrangement allows using a wider input waveguide width compared to a standard AWG of similar spectral performance. The interleaved AWG, however, produces (in the combiner free propagation region) its distinct spatial Talbot effects, and the superposition of these Talbot patterns yields a spatial Moiré pattern. Signal retrieval from the complicated Moiré-Talbot pattern requires complicated numerical procedures.